1. Field of the Invention
The present invention refers to a method of bending sheet metal pieces by means of a bending apparatus comprising a bending bar, a bending bar holder, a bottom die cooperating with said bending bar and including a die bottom which is adjustable in height, and means for receiving and supporting said bottom die, whereby said bottom die and said means for receiving and supporting said bottom die, respectively, or said bending bar and said bending bar holder, respectively, are provided with a number of plunger-cylinder units arranged side by side.
Further, the invention relates to an apparatus for bending sheet metal pieces comprising a bending bar, a bending bar holder, a bottom die cooperating with said bending bar and including a die bottom which is adjustable in height, and means for receiving and supporting said bottom die, whereby said bottom die and said means for receiving and supporting said bottom die, respectively, or said bending bar and said bending bar holder, respectively, are provided with a number of plunger-cylinder units arranged side by side.
2. Prior Art
For bending sheet metal pieces, use can be made of folding presses which, generally, comprise a movable bending bar and a bottom die placed in a bottom die holder. During bending of a sheet metal piece, high forces are required to bend the sheet metal piece, depending on the thickness of the sheet metal piece. Thereby, the thus resulting high working pressures require a heavy construction of the folding press in order to prevent a deflection of the frame of the press. However, it became apparent that deflection of the press frame cannot be avoided. It is obvious that such deflections affect the precision of the bending process. Consequently, attempts were made to avoid or to compensate such possible deflections.
One proposal of Applicant was to use a work table which rests on a supporting element having the same length as the work table. The supporting element is vertically shiftably beared in a bottom part on an oil cushion. Thereby, a possible deflection of the press frame can be compensated for during bending heavy sheet metal pieces.
A further embodiment of such a folding press, based as well on a proposal of the applicant, consisted in placing the bottom die holder on a plurality of plunger-cylinder units lined up side by side. This method also allows a compensation for possible deflections of the press frame when bending heavy workpieces.
However, if the work piece to be worked is shorter than the available length of the work table, particularly considerably shorter, the work piece is too much pressed at its ends, and inexactnesses will arise there, even if the work table rests on an oil cushion or on a plurality of plunger-cylinder units.
A more precise working is provided by a folding press in which the elastically yieldingly supported tool itself or its support is composed by a number of partial elements which are arranged side by side and which are individually vertically displaceable. If under load, only those partial elements which are directly or indirectly supporting the work piece are displaced towards the elastic support. With such a folding press it is favourable to support the bottom die holder by a number of plunger-cylinder units arranged side by side, whereas the bottom die is provided with an adjustable die bottom so that by adjusting the height position of said bottom any desired bending angle can be preselected.
Further, it has been proposed to design a folding press in such a manner that the upper or the lower tool assembly, respectively, is supported by the plungers of a plurality of vertical hydraulic supporting cylinders which are secured to the upper or the lower tool assembly, respectively, and are distributed over the length of the folding press. Said supporting cylinders are linked so as to form a closed hydraulic system.
However, this design has several disadvantages. Particularly, a precise bending of a workpiece to different bending angles is not possible without more ado. If the sheet metal piece to be bent is shorter than the bending bar or the bending bottom die, respectively, the stability of the bending process is endangered, and single-sided loadings will occur.